Make __type introspection field nullable per spec

* Updated `TypeMetaFieldDef` to return nullable `__Type` (StructNullable).
* Changed resolver to use `Seq.vtryFind`, returning null for unknown types.
* Added test to verify `__type` returns null for unknown type names.

Author: xperiandri

src/FSharp.Data.GraphQL.Server/Planning.fs

@@ -27,7 +27,7 @@ let TypeMetaFieldDef =
     Define.Field(
         name = "__type",
         description = "Request the type information of a single type.",
-        typedef = __Type,
+        typedef = StructNullable __Type,
         args = [
             { Name = "name"
               Description = None
@@ -38,8 +38,8 @@ let TypeMetaFieldDef =
         ],
         resolve = fun ctx (_:obj) ->
             ctx.Schema.Introspected.Types
-            |> Seq.find (fun t -> t.Name = ctx.Arg("name"))
-            |> IntrospectionTypeRef.Named)
+            |> Seq.vtryFind (fun t -> t.Name = ctx.Arg("name"))
+            |> ValueOption.map IntrospectionTypeRef.Named)
 
 /// Field definition allowing to resolve a name of the current Object type at runtime.
 let TypeNameMetaFieldDef : FieldDef<obj> =

tests/FSharp.Data.GraphQL.Tests/IntrospectionTests.fs

@@ -328,6 +328,19 @@ let ``Core type definitions are considered nullable`` () =
         empty errors
         data |> equals (upcast expected)
 
+[<Fact>]
+let ``__type must return null for unknown type name`` () =
+    // Spec: `__type(name: String!): __Type` (nullable), so unknown type names must resolve to null.
+    // https://spec.graphql.org/draft/#sec-Schema-Introspection.Schema
+    let root = Define.Object("Query", [ Define.Field("onlyField", StringType) ])
+    let schema = Schema(root)
+    let query = """{ __type(name: "DefinitelyMissingType") { name kind } }"""
+    let result = sync <| Executor(schema).AsyncExecute(query, getMockInputContext)
+    let expected = NameValueLookup.ofList [ "__type", null ]
+    ensureDirect result <| fun data errors ->
+        empty errors
+        data |> equals (upcast expected)
+
 type User = { FirstName: string; LastName: string }
 type UserInput = { Name: string }